1. Field of the Invention
The invention relates to conductive plastics and somewhat more particularly to a method of producing conductive plastic materials from a plurality of components having low conductivity tolerances by, for example, addition of conductive materials to select plastics.
2. Prior Art
It is known to admix plastics with conductive additives in order, among other things, to produce antistatic parts. To achieve this, lampblack, for example, is added to a plastic in a specified ratio and the admixture is formed, as by extrusion, into a desired part. The resultant surface resistance or, respectively, volume resistance, differs from a given part to another different part, depending on the intend use of such parts. Surface resistance above 10.sup.10 .OMEGA./.quadrature. and below 10.sup.4 .OMEGA./.quadrature. are attainable by means of a fixed material formula. However, it is not possible to work with a fixed material formula in the range lying between the above noted resistance values because slight differences in the quality of the components as well as minor fluctuations in the extrusion parameters lead to intolerable variations in resistance values.
Heretofore, the dosing relationship between a plastic material and a conductive material has been gravimetrically or volumetrically defined. However, this is disadvantageous because the properties of the individual components or materials are left out of consideration and, consequently, the final results vary greatly. Such results depend, for example, on the conductivity and, under given conditions, on the distribution of the particle size of the lampblack utilized.
At present, there is no possible way of measuring the conductivity of lampblack in the dosing step and to utilize such measured value for controlling the resistance of a formed part. Heretofore, as already mentioned, measuring or dosing of lampblack has been gravimetric or volumetric and the electrical resistance property was not checked until the finished injection-molded parts were available.
An apparatus for dosing a plurality of individual materials in a mixed material charge intended for processing in an extruder has been described and illustrated in German Offenlegungsschrift No. 28 38 110 and such apparatus may be used in the practice of the invention. This apparatus operates analogous to a differential control, so that a high degree of precision with respect to temperature and viscosity of the extruded material can be achieved.